This application relates to a load sense hydrostatic steering system which directs fluid to a load sense hydrostatic vehicle steering circuit, and to an auxiliary circuit. More particularly, the application relates to a system with a variable displacement pump, and a priority valve for controlling fluid flow from the pump to the steering circuit and the auxiliary circuit.
A. Load Sense Hydrostatic Steering Systems - In General PA0 B. Load Sense Hydrostatic Steering Controllers PA0 C. Auxiliary Circuit With Load Sense Devices PA0 D. Use of Shuttle Valve Logic To Isolate Parts of Load Sense Systems PA0 E. Auxiliary Circuits With Solenoid Actuated Bypass Valve
A load sense hydrostatic steering system can be useful in an off-the-road vehicle such as a farm combine. The system can make use of fluid which is available from a pump to steer the combine, and to operate one or more auxiliary devices on the combine (e.g. reels, platforms, unloaders, etc.) with the fluid which is not needed for steering.
In a load sense hydrostatic steering system for a farm combine, it is well known to utilize a priority valve to direct fluid from a pump to a steering circuit for steering the combine, and to direct fluid which is not needed for steering to an auxiliary circuit for operating auxiliary devices on the combine It is also known to provide the system with a pump whose displacement is variable, and to control the displacement of the pump so that the pump (i) delivers a standby flow and pressure to the system when there is no steering and when none of the auxiliary devices are operating, and (ii) increases flow and pressure delivered to the system in order to meet the needs of the steering circuit and/or the auxiliary circuit when the vehicle is being steered and/or when one of the auxiliary devices is being operated.
As shown by U.S. Pat. Nos. 3,931,711 and 4,079,805, in a load sense hydrostatic steering system the steering circuit has a steering controller with a control valve that responds to movement of a steering wheel to (i) direct fluid flow to the steering motor, and (ii) provide a pressure signal indicating that there is a demand for fluid for steering. During steering, as the control valve directs fluid to the steering motor, the magnitude of the pressure signal varies with variations in the rate at which the operator steers (operator demand) and the amount of resistance encountered by the vehicle's wheels (steering load). When the system has a variable displacement pump and a priority valve, the pressure signal can be used to control the displacement of the pump and the position of the priority valve to insure that the demands of the steering circuit will be satisfied, as is well known to those in the art.
In many steering systems, a pilot flow originates in the steering controller and is used to provide the pressure signal for controlling the pump and/or the priority valve. U.S. Pat. Nos. 3,931,711 (FIG. 8), 4,079,805 (FIG. 3) and 4,167,893 show such controllers. In an improved system, shown in U.S. patent application Ser. No. 243,497, assigned to the assignee of this application, a pilot circuit originates outside the controller, and a fluid pressure signal from that pilot circuit controls a priority valve. If the pump has a variable displacement, the fluid pressure signal acts on a flow compensator valve to control the displacement of the pump. In the pilot circuit, fluid (preferably oil) is directed through the steering controller to a reservoir when there is no steering. When an operator begins to steer, the control valve immediately restricts the flow of pilot fluid to the reservoir. A fluid pressure spike is produced in the pilot circuit even before there is actual fluid flow to the steering motor. The fluid pressure spike anticipates a demand for fluid for steering, and acts on the priority valve (and the flow compensator valve) to rapidly urge those elements to positions in which the system will satisfy the steering demand.
In a load sense hydrostatic steering system, the auxiliary circuit often includes one or more devices which, as they are operated, produce a load sense pressure signal which varies in accordance with the load encountered by the device. Such an auxiliary system has been sold by Control Concepts, Inc., Newtown, Pa., and has been used for operating different devices on a farm combine. The load sense pressure signal produced by operation of a device represents a demand for additional fluid flow and pressure for operating that device. Such a device would be known as a load sense auxiliary device.
When an auxiliary circuit has several load sense auxiliary devices, the devices may produce different load sense pressure signals, indicating different demands for fluid flow and pressure. In the system sold by Control Concepts, Inc., the auxiliary circuit determines which load sense pressure signal evidences the highest demand, and transmits that signal to the pump for increasing the flow and pressure delivered by the pump. Further, in the system sold by Control Concepts, Inc., the pressure lines which transmit the load sense pressure signals for the auxiliary load sense devices are bled to a reservoir through respective bleed orifices, so that the flow and pressure delivered by the pump may reduce when the demand for fluid by the auxiliary devices ceases.
In load sense systems where the displacement of a pump is controlled by pressure signals from several devices, it is well known to use shuttle valve logic to make the system operate properly. Shuttle valves are valves which receive several pressure input signals, and transmit the highest pressure signal, while blocking that signal from the other inputs. U.S. Pat. No. 4,089,169 shows a load responsive system which uses shuttle valves.
In a load sense steering system, shuttle valves can compare the pressure signal from a steering circuit and the pressure signals from the load sense devices in the auxiliary circuit. The shuttle valves transmit the pressure signal evidencing the highest need for fluid for controlling the displacement of the pump. Further, the shuttle valves effectively isolate appropriate parts of the auxiliary circuit from each other and from the steering circuit. Thus, if an auxiliary circuit has separate bleeds associated with the load sense auxiliary devices (e.g. the Control Concepts, Inc. system), the bleeds do not interfere with the proper operation of the system.
Finally, in the auxiliary load sense system sold by Control Concepts, Inc., Newtown, Pa., a solenoid actuated bypass valve can be provided in the auxiliary circuit. The valve is biased to an inactive position, and has a solenoid which is energized to shift the valve to an active position either selectively, or under certain system conditions (i.e. fluid temperature). When the bypass valve is in an active position, it operates to short circuit the pump pressure line to the auxiliary pilot line and causes the pump pressure to elevate to its maximum pressure, and operate some auxiliary devices at constant maximum pressure. This operational mode is known in the art as an override of the load sense flow controller to cause constant pressure closed center operation of the system.